The present invention relates to inserts of the blind nut type which are permanently installed in a panel with access to the panel from one side only. More specifically, it applies to a blind nut which provides flush clinch-type attachment by receiving the cold flow of metal from the panel.
Blind nuts are a class of inserts well known in the insert industry which provide a threaded element to a thin panel such as a sheet of metal which is of insufficient depth and/or hardness to support threads directly in the sheet. This type of a nut is called xe2x80x9cblindxe2x80x9d because it can be applied in situations where there is access to only one side of the panel. Typically the nut is gripped to the panel between a flanged head which lies against one side of the panel and collapsed bulges or bulbs which abut the opposite side of the panel. The bulbs are created by deforming an intermediate tubular portion of the nut shank of reduced diameter (hereinafter xe2x80x9cdeformable portionxe2x80x9d) adjacent to an internally threaded end by a compression tool mandrel threaded into the nut. After the nut is installed, the mandrel is unthreaded from the nut leaving the nut/panel assembly ready to receive a threaded screw or bolt for attachment of other parts to the panel. There are also externally threaded blind inserts in which a permanently attached stud replaces the function of the mandrel as the means for applying the compression load to the end of the insert.
The performance of a blind nut/panel assembly may be described by two distinct axial strength considerations of attaching a mating threaded insert. The first is the clamp load capacity, which is a measure of the amount of axial load that can be applied when the mating threaded fastener is tightened. Clamp load capacity is a function of the thread-stripping strength and the support strength of the threaded area. Considering the shape of the cross section of the installed insert, this support strength is a combination of the bulb shear strength and the compressive strength of the deformable portion within the panel. The second strength consideration is the working load capacity which is a measure of the load required to separate the attached part from the panel containing the blind insert. Considering the shape of the cross section of the installed insert, the bulb is the only element preventing the insert from pulling out of the panel and hence the working load capacity is simply the bulb shear strength. Because the additive effect of the deformable portion compressive strength is not present for the working load, the working load capacity will always be less than the clamp load capacity.
Another consideration is the type of installation tool that may be used. Tools to install blind nuts are of two basic types: spin-pull tools and spin-spin tools. Spin-spin tools create compressive load in the collapsible region of the insert by rotation of a mating threaded member. Spin-spin tools are inexpensive, light-weight and simple to set up and use and are therefore generally preferred. Because the upset force they can develop is limited, they generally cannot be used to install parts with thicker walls. Spin-pull tools spin the mating threaded member into position and then apply an axial pulling force before spinning in the opposite direction to remove the threaded member from the nut. Spin-pull tools are more complex and consequently more expensive and heavier than spin-spin tools. They are also more difficult to set up and require more maintenance. The upset load or the force required to form the panel gripping bulb therefore often determines which of these two common types of installation tools can be used. It is therefore desirable to provide a blind nut insert with the greatest working load capacity and with an upset load that can be achieved by the limited upset force provided by a spin-spin type installation tool.
A desired performance characteristic of blind nuts is a large grip range. That is, the ability for the same part to attach to panels of various thickness. Hereinafter, the term xe2x80x9cgripxe2x80x9d will be used to mean the portion of the shank deformable portion that lies within the panel hole between the primary bulb and the insert head.
In certain applications, there is a need for a blind nut which can be installed flush with the access side of the panel. This may require countersinking the panel to receive the head of the nut so that the face of the head is flush with the front of the panel, however this is undesirable since countersinking the panel requires an additional manufacturing operation. To overcome this problem, a broaching-type blind nut has been devised which provides flush attachment in which the nut does not have a head but instead has a serrated end portion with teeth that secure the blind nut to the workpiece in a non-rotating relationship without the need for a countersink in the panel. Such an insert is disclosed for example in U.S. Pat. No. 3,948,142 issued to McKay et al. In order to accommodate the broaching attachment, a cavity in the interior of the nut receives the inward collapsing deformable wall of material into the body of the nut. One of the problems with this type of attachment, however, is that there is no positive structure on the insert to prevent the insert from pushing out of the sheet. A push-out load equal to the broaching load will cause it to broach all the way through the panel and become dislodged.
It is also known in the art to utilize a self-clinching type attachment for flush-mounted blind nuts. A self-clinching type blind nut is shown, for example, in U.S. Pat. No. 3,215,026 issued to Davis. Clinch-type attachment requires a much higher installation clamp load than broaching attachment since it depends on the cold flow of the deformed metal panel. Therefore, within the range of the shear strength of the known blind nut materials and construction, this insert is limited to metal panels of relatively soft material. Attempts to increase shear strength of the nut by increasing the strength of the nut material works against the nut""s ability to properly collapse against the backside of the attached sheet given the limited working force of installation tools. Also, attempts to structurally increase the strength of the clamp load of the blind nut without increasing the material strength are shown, for example, in U.S. Pat. No. 2,324,142 issued to Eklund in which the collapsible section of a blind rivet is provided with a second reinforcing bulb. However, as disclosed, this structure requires applying pairs of grooves to the outer surface of the collapsible portion of the nut shank which does not provide the necessary bulb shear strength for the higher working loads required by clinch-type attachment into harder metal sheets.
It is further known to create an area of weakness in the region of a blind nut deformable portion by annealing to create a zone of reduced hardness along the length of the deformable portion. When the insert is compressed, a bulb forms in the annealed area. This method of altering a blind nut metallurgy for bulb production is shown, for example, in U.S. Pat. No. 4,826,372 issued to Kendall.
Despite the efforts in the art, a blind nut which can be installed flush with the access side of the panel and which displays a wide grip range that can be installed by a spin-spin tool has not yet been achieved. There is therefore a need in the art to provide a blind nut with a clinch-type flush attachment capable of use in sheets of harder metal. There is a further need to provide a clinch-type blind nut for harder metal panels which can be installed with a spin-spin type tool and which provides a wide grip range for installation into panels of varying thickness.
To meet the needs in the art described above, the present blind nut has been devised. It provides a blind nut with a relatively thin wall and an increased deformable shank length with multiple bulb producing capability. The bulbs may be formed by providing the inside or outside surface of the insert with additional material in the area of the deformable portion of the shank. Alternatively, selectively hardened collars may be formed in the material in the deformable region by heat treatment such as selective annealing.
Because the wall thickness is relatively thin, the present insert can be installed with a spin-spin tool and provides a wide grip range. When the present insert is installed, two bulbs will form. By design, the primary bulb in contact with the sheet will always be the same or larger in diameter than the secondary supporting bulb that forms adjacent to the threaded end region. Because two bulbs are formed, the bulb shear strength will be proportional to four times the wall thickness rather than being proportional to two times the wall thickness when only one bulb is formed.
By controlling the axial position of the multiple bulb producing feature of the invention, the order in which the multiple bulbs form can be controlled. In most designs, the bulb adjacent the panel will be formed first. In one embodiment, the compressive strength of the part inside the panel mounting hole is reduced by increasing the clearance between the outside diameter and the mounting hole. This clearance results in bending of this area when the bulb forms adjacent to the panel. The presence of this initial bending facilitates additional bending as the head is embedded.
In another embodiment, the multiple bulb producing capability of the insert can be provided by changing the metallurgical properties of selected localized areas of the insert by either selective hardening or selective annealing. Alternatively, a reinforcing rib may be added to the outside diameter of the part to create a localized area of increased strength, on either side of which bulbs are formed as will be further described herein.
More specifically, the invention comprises a threaded blind insert having a shank with an axial bore therethrough having a head at one end and a threaded region of the bore at the opposite end, the shank having a wall line between an outer surface thereof and the bore. A deformable portion of the shank is created by an area of reduced diameter located axially along the outer surface of the shank A reinforcement collar integral with the shank is located at approximately the middle of the area of reduced diameter and provides a greater wall thickness at that point to create bulges on either side of the collar as the insert is compressed when it is clamped to a panel. The insert further includes a flange located adjacent the underside of the head. The flange has an outer diameter greater than the diameter of the shank but less than the diameter of the head. The head includes a knurl or other non-round feature such as a polygonal shape. The insert attaches to a panel having a circular aperture with a diameter slightly less than the diameter of the flange which is fitted within the aperture. By these dimensions, a substantial annular gap is provided which lies between the shank and the sidewall of the aperture. As the insert is compressed, bulges form on either side of the collar and provide a reactive force against a backside of the panel which clamp the panel between the bulges and the head of the insert. The reaction forces of the bulges is sufficient to embed the head and the knurl into the panel such that the final installation provides a flush attachment with the front surface of the panel. Alternatively, the reinforcement collar can be replaced by a band of greater hardness of the deformable wall which is created by heat treating the insert. This can be achieved by annealing portions of the deformable portion on either side of the band or heat treating the area of the band to create an area of greater hardness or annealing the material on opposite sides of the band to soften the material in those regions.
Thus, the present invention provides the following advantages. First, it provides increased bulge strength relative to wall thickness and upset load. Secondly, it allows a flush installation on the side from which the insert is installed without special hole preparation. Thirdly, as further explained herein, it provides increased torsional holding strength in a round mounting hole. And finally, the invention provides increased grip range compared to blind inserts using heavier wall thickness to increase bulb shear strength. Other objects and advantages will be apparent to those of skill in the art from the following drawings and detailed description of the preferred embodiment.